I am using Clearblue fertility monitor for every natural cycle. I found some drawback of the monitor and would like to point it out. The following is my personal experience. Let me know if you think differently.

In my understanding, the best time to BD is two days before ovulation. BDing on the day of ovulation/the day after ovulation also help to cover all the bases, since the egg can live up to 24 hours.

Clearblue fertility monitor tests two hormones: 1. LH 2. Estrogen. It asks you to test the first morning urine. But usually LH surge is in the early AM and takes a few hours to get into the urine. So if you use first morning urine, you will most likely detect the LH surge the next morning. The fertility monitor will show "peak". If you BD on the Peak day at night, it is already kind of late. Because very likely you have ovulated on the same day (24-48 hours/average 36 hours after the LH surge). So it is very important to BD before the "Peak" day. Usually the monitor gives two "High" days. If you start BDing on the "HIGH" days, you are o.k.

But some cycles the monitor showed no "high" days before "peak" day. If i rely on the monitor, I will completely miss my opportunity in that cycle.

So here's what I do now. I take the first morning urine as directed and conduct the test. Then I do a manual test in the PM and check the LH line manually. If I see a second line (LH) darker than the first line (estrogen), I start BDing. It gave me an extra day. But please be aware, before the LH surge, you can see a light second line, just not as dark as the first line. It can sometimes be confusing. I monitor my mucus and basal temperature too. Usually I see fertile mucus 2-3 days before LH surge. I become nervous because I don't want to start BDing too early because it will deplete my hubby's supply. But I don't want to be too late. So the manual test become critical for me. I start once I notice the second line is as dark as the first line (usually the day before LH surge) or darker than the first line (the day of LH surge. the monitor usually reads HIGH). Then I monitor basal temperature to confirm ovulation.

It took me quite a few cycles to figure out this way. Just like to share. let me know if you have different opinions or experience to share.